Chromia scales

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

A.C.S. Sabioni, A.M. Huntz, J. Philibert, B. Lesage, C. Monty. Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr203 // J.Mater. Sci.-1992.- V.27, No. 14.- P.4782-4790. 

J. H. Park, Role of Yttrium in Enhanced Adhesion of Chromia Scale to Chromium, Materials Letter, 8(10), 1989, pp.405-408. 

K. Przybyiski, A. J. Garratt-Reed, and G. J. Yurek, Grain-boundary Segregation of Yttrium in Chromia Scale, Journal of Electrochemical Section, 135,1988, p.509. 

Hydrogen permeation tests on ferritic stainless steels indicated that hydrogen can diffuse through the alloys, though the permeation was drastically decreased by formation of chromia scale on the alloys. - The mechanisms by which the presence of hydrogen or protons at the air side affects the oxide scale structure and growth are not clearly understood at this time. Several mechanisms have been proposed to tentatively explain the observed anomalous oxidation behavior. ... 

In Wei et al. s work,51 after 7 days of oxidation at 750°C, a relatively thick layer ft-5-1 Opm) of Cr-rich oxide formed under the Co-Mn and Cu-Mn spinel layers. Although such thick chromia scales are likely vulnerable to break down in long term or at higher temperatures, no anomalous oxidation behavior was reported. Similarly, in Deng et al. s work55 from the same research group, 5-7-pm-thick Cr-rich oxides formed under the Co spinel layer. 

The process described above in which a solute oxidizes preferentially to the parent element and forms a continuous layer on the surface is referred to as selective oxidation. The selective oxidation of elements which form a slowly growing, protective layer is the basis for the oxidation protection of all alloys and coatings used at high temperature. The only elements which consistently result in protective scales are Cr (chromia scale), Al (alumina scale), and Si (silica scale). Therefore, much research has been directed at finding alloy and coating compositions, which meet other property (e.g., mechanical) requirements and also form one of these scales. 

Of course, if the protective scale of chromia or alumina is not penetrated by SO2, sulphide cannot form at the scale-metal interface. This was found for Ni-20 wt% Cr, Co-35 wt% Cr and Fe-35 wt% Cr alloys exposed to pure SO2 at 900 °C and emphasizes the resistance of a chromia scale to permeation. On the other hand, alloys in the Fe-Cr-Al, Ni-Cr-Al and Co-Cr-Al systems were exposed to atmospheres in the H2-H2S-H2O system. These atmospheres had compositions that supported the formation of chromia or alumina together with the sulphides of Fe, Ni and Co at the scale-metal interface. In these cases, a protective layer of chromia or alumina that formed initially was penetrated by sulphur to form iron, nickel, and cobalt sulphides at the scale-metal interface. Furthermore, iron, nickel, and cobalt ions apparently diffused through the oxide layer to form their sulphides on the outside of the protective scale. Thus the original protective scale was sandwiched between base-metal sulphides. 

Interestingly, in addition to outward diffusion of chromium from the native scale into the coating there docs appear to be some diffusion of Mn (and Co) fiom the coating inward, into the scale. This leads to the possibility of the MCO coating doping the native chromia scale and leading to an improvement in the conductivity of the scale which in turn could improve ASR performance of the coated interconnect. 

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